Transcript magnetic field - the SASPhysics.com

Electromagnetism
(and a bit of energy and
electricity)
6.2 understand that magnets repel and attract other magnets and attract magnetic substances
6.3 describe the properties of magnetically hard and soft materials
6.4 understand the term ‘magnetic field line’
6.5 understand that magnetism is induced in some materials when they are placed in a
magnetic field
6.6 describe experiments to investigate the magnetic field pattern for a permanent bar magnet and that
between two bar magnets
6.7 describe how to use two permanent magnets to produce a uniform magnetic field pattern.
6.8 understand that an electric current in a conductor produces a magnetic field round it
6.9 describe the construction of electromagnets
6.10 sketch and recognise magnetic field patterns for a straight wire, a flat circular coil and a
solenoid when each is carrying a current
6.11 understand that there is a force on a charged particle when it moves in a magnetic field as
long as its motion is not parallel to the field
6.12 understand that a force is exerted on a current-carrying wire in a magnetic field, and how this
effect is applied in simple d.c. electric motors and loudspeakers
6.13 use the left hand rule to predict the direction of the resulting force when a wire carries a current
perpendicular to a magnetic field
6.14 describe how the force on a current-carrying conductor in a magnetic field increases with the
strength of the field and with the current.
6.15 understand that a voltage is induced in a conductor or a coil when it moves through a magnetic
field or when a magnetic field changes through it and describe the factors which affect the size of the
induced voltage
6.16 describe the generation of electricity by the rotation of a magnet within a coil of wire and of a coil
of wire within a magnetic field and describe the factors which affect the size of the induced voltage
6.17 describe the structure of a transformer, and understand that a transformer changes the
size of an alternating voltage by having different numbers of turns on the input and output
sides
6.18 explain the use of step-up and step-down transformers in the large scale generation and
transmission of electrical energy
6.19 know and use the relationship between input (primary) and output
(secondary) voltages and the turns ratio for a transformer:
6.20 know and use the relationship: input power = output power
VP IP = VS IS for 100% efficiency
4.2 describe energy transfers involving the following forms of energy: thermal(heat), light, electrical,
sound, kinetic, chemical, nuclear and potential(elastic and gravitational)
4.16 describe the energy transfers involved in generating electricity using:
Wind, water, geothermal resources, solar heating systems, solar cells, fossil fuels, nuclear power
4.17 describe the advantages and disadvantages of methods of large scale electricity
production from various renewable and nonrenewable resources.
What is a magnet?
• Princeton University definition:
• Magnet: (physics) a device that attracts iron
and produces a magnetic field
• Magnetic: of or relating to or caused by
magnetism; "magnetic forces" ;having the
properties of a magnet; i.e. of attracting iron
or steel
• Magnetism: attraction for iron; associated
with electric currents as well as magnets;
characterized by fields of force
Are we any wiser?
7 quick questions...
1. What do we call the region around a magnet
where some materials experience a force?
2. Which materials experience a magnetic
force?
3. What do we call the ends of a magnet?
4. Why?
5. How do you magnetise something?
6. What do you get if you cut a magnet in half?
7. What causes magnetism?
What magnets do
• Magnets exert a force on
– Other magnets
– Objects containing iron, cobalt and nickel
(ferromagnetic materials)
• Magnets always attract ferromagnetic
materials
• Magnets can attract or repel other
magnets
– Depends on polarity
Bar Magnets
• Bar magnets are permanent magnets made
from magnetic materials.
• 2 poles
North (seeking)
pole
South (seeking)
pole
– A suspended magnet will align with the Earth’s
magnetic field
• Unlike poles attract, like poles repel.
• How can you test if a piece of metal is a
magnet?
– You can only show that an object is a magnet if it
repels a known magnet.
Magnetic Fields
• Magnets exert a force at a distance, no contact is
needed.
• The space around a magnet where a magnetic
force is felt is called a magnetic field.
• We represent a magnetic field with field lines.
Magnetic field lines are not “real”
(like contour lines on a map)
• Field lines:
–
–
–
–
go from North to South
are closed loops
never cross
are most concentrated where the field is strongest
Investigating Fields
• A little magnet which is free to move will
align itself with an applied magnetic field
• We can use plotting compasses or iron
filings (which will act like little magnets) to
find the shape of field patterns
Your turn…
• Using plotting compasses or iron filings,
draw the magnetic field patterns for:
1. A single bar magnet
2. Two magnets arranged as follows
(a)
(b)
Ν
S
Ν
S
S
N
Ν
S
(c)
(d)
N
S
N
N
2 magnets, opposite poles facing
Single bar magnet
2 magnets, same poles facing
Field lines are also
called flux lines.
Uniform Field
• The magnetic field between a N and S
pole is almost uniform.
• We can get closer with slab magnets.
• Can do even better with electromagnets
N
S
Questions
• Below is a bar magnet and a compass. Label the poles
of the magnet and draw the field line on which the
compass lies.
Ν
S
• Identify the poles A-F below:
N
N
S
S
N
S
Making magnets
• A piece of magnetic material can be
magnetised by placing it in a magnetic field
(due to another magnet, or created
electrically).
• If it is a magnetically hard material (eg
steel), it will retain its magnetism.
• Permanent magnet
• If it is a magnetically soft material (eg iron),
it will demagnetise as soon as the field is
removed.
• Temporary magnet
Earth’s Magnetic Field
• The Earth has its own magnetic field,
similar to that of a bar magnet.
A magnet free to
rotate will align
with the Earth’s
field
Electricity and magnetism
• Before the beginning of the
19th century, magnetism
and electricity were thought
to be separate phenomena.
• In 1820 the Danish scientist
Hans Christian Ørsted
noticed that when a
compass was placed near a
wire carrying an electric
current, the compass
needle was deflected.
Magnetic field around a wire carrying
current
• Direction of field given by Maxwell’s Right
Hand grip rule (or think of a screw thread).
Magnetic field due to a loop of
wire
• By applying RH
grip rule around the
wire we find the
resulting magnetic
field.
Field due to a solenoid
• A solenoid is an extended coil of wire.
• The magnetic field pattern of a solenoid carrying current
is very similar to that of a bar magnet.
Electromagnets
• A solenoid’s magnetic field can
be made stronger by
– Increasing the current
– Increasing the number of turns
of wire
– Wrapping the turns around a
magnetically soft core
• Combining these can produce
extremely powerful (and
switchable) electromagnets
– Video at bottom of page
Uses of electromagnets
• Smaller electromagnets are used in bells
and switches
Force on a current in a magnetic
field – The Motor Effect
• Why do we get movement?
– Because a force is acting
(called the Motor Effect).
– NOT because of magnetic
attraction (wire is nonmagnetic material)
• Direction of force relative to:
– Current?
– Magnetic field?
• Perpendicular to both
The Motor Effect
• We can increase the force produced by:
– Increasing the magnetic field strength
– Increasing the current in the wire
– Increasing the number of turns of wire in the field
(add coils)
– Wrap the wire coils around a soft iron core
• Only a wire crossing field lines experiences a
force, not wires parallel to field lines
– Greatest force when field and current are at 90
degrees
• A conductor carrying a
current in a magnetic field
experiences a force
perpendicular to both.
• This is known as the
MOTOR EFFECT.
• The direction is given by
Fleming’s left hand rule.
Motion
Fleming’s Left Hand Rule
The Catapult Field
• The force on the wire is
due to the interaction of
the fixed magnetic field
and the field due to the
current flowing in the wire.
• Where the two fields are
in the same direction they
reinforce each other
• Where they are in
opposition they produce a
weaker field.
A simple electric motor
• Rotation due to interaction of coil’s
magnetic field and fixed magnet’s field
Motor Effect Questions
• A wire carries a current horizontally between the poles of
a magnet, as shown below. The direction of the force on
the wire is:
• A from N to S
• B from S to N
• C opposite to the current direction
• D In the direction of the current
current
• E vertically upwards?
N
S
Motor Effect Questions
• In the figure below, AB is a copper wire hanging from a
pivot at A and dipping into mercury in a copper dish at B.
It is suspended between the poles of a powerful magnet.
• (a) Copy the diagram and add the magnetic field lines
• (b) Mark in the direction of the conventional current
• (c) What will happen when the switch is closed?
A
N
S
B
Loudspeaker
• The loudspeaker uses
the motor effect to
change electrical
energy to sound.
– The varying electrical
signal changes the
field due to the coil.
– This interacts with the
permanent magnet,
moving the coil and
attached cone.
Electric motor operation
Need a special
electrical contact to
reverse current
direction every half
turn:
COMMUTATOR
Commutator in action
DC Electric Motor details
• What you need to know...
• The COMMUTATOR
reverses the direction of
the current in the coil
every half turn,
maintaining the direction
of the couple and keeping
the motor turning
DC Electric Motor details
• What you need to know...
• The direction of rotation
reverses if:
– You reverse the field OR
– You reverse the power
supply
• What happens if you
reverse both?
DC Electric Motor details
• What you need to know...
• The motor spins faster if
you:
– Make magnet stronger
• Some real motors use
electromagnets
– Increase the coil current
– Increase number of coils
– Give coils an iron core
Currents don’t need wires!
• A beam of electrons in a
vacuum tube is also a current
• Such a beam of charged
particles is deflected by a
perpendicular magnetic field.
– No effect if the field is parallel to
direction of motion
Electromagnetic induction
• Any conductor experiencing a changing
magnetic field (or moving across a steady
magnetic field) will have a p.d. induced
across it.
• If a closed circuit is made, a current will
flow.
• This is the basis of almost all electricity
generation.
Inducing a bigger voltage
• The induced voltage is
bigger if we:
– Move the wire quicker
– Increase the magnetic
field strength
– Increase the length of
wire in the field by
wrapping it into a coil
Electromagnetic induction
Using a coil of wire increases the voltage produced
Electromagnetic induction
• The direction of the induced voltage can
be reversed by:
– Reversing the magnet
– Moving the magnet in the opposite direction
Electromagnetic induction
• The size of the induced voltage can be
increased by:
– Moving the magnet faster
– Increasing the number of turns on the coil
– Using a stronger magnet
– Larger area coil
The faster magnetic field lines
are cut by the wire, the bigger the
induced voltage and current
Generators / dynamos
• To generate a continuous voltage we need
a constantly changing magnetic field.
• This is achieved by rotating a magnet in or
near a coil of wire.
• An ALTERNATING
CURRENT is produced.
• This is how mains electricity is generated
Alternating output
Magnet position
• Output voltage is continually changing, and periodically
changes direction (negative value on graph)
Dynamos
• Simplest to engineer with stationary coil and rotating magnet
Alternating and direct current
• A battery provides a direct
current
– Electrons always flow in one
direction
• A generator provides an
alternating current
– Direction of current changes over
time, electrons flow back and forth
• Both types of current are able
to transfer electrical energy
Why alternating current (AC)?
• It is easy to generate
• It is easy to transport over long distances with
low power loss
• Many applications (light, heating etc) work fine
with AC
• The mains supply to our homes is 230V AC
• For applications which need DC, we can build
electric circuits to convert the power supply.
Or rotate the coil...
• Wire just needs to cut
field lines, it doesn’t
matter which bit is
moving
• Slip rings are
contacted to ends of
the coil and rotate with
it
• Brushes slide against
rotating rings and
provide electrical
contact
• Output is AC voltage
– Same machine as an
AC motor
• See here for an animated version
Increasing generator output
• In a real high power generator this is done
by:
– Using a stronger rotating electromagnet
– Rotating the magnet faster
• But mains electricity has a defined frequency (50 Hz)
– Using many fixed coils with more turns
– Putting an iron core inside the fixed coils
Battersea Power Station, 1933
Rotating
electromagnet
Coils of wire
Mutual induction
• Remember electromagnets?
– When a current flows in a coil of
wire a magnetic field is produced
• If an alternating current flows,
then an alternating magnetic
field is produced
• If a second coil of wire
experiences this changing
field, a voltage is induced in it
• Run this simulation and click in
the transformer tab
Induction cooker
• AC in a coil induces a
changing magnetic field
• This in turn induces
alternating currents in
the metal pan
• Resistive heating
causes the pan to get
hot, cooking the food
Rechargeable toothbrush
Transformers
• A transformer consists of two coil mounted on a common
iron core
• An alternating current flowing in the primary coil
produces a changing magnetic field
• The iron core concentrates the field through the centre of
the secondary coil
• The alternating magnetic field induces an alternating
current in the secondary coil
• This happens even though there is no direct electrical
connection between the two coils
Transformers only work with AC. DC does not get through!
Transformer action
• A transformer can change the voltage.
• The size of the voltage induced in the secondary coil
depends on the number of turns in the primary and
secondary coils and also the voltage applied to the
primary coil.
Voltage across secondary coil
number of turns on secondary coil

Voltage across primary coil
number of turns on primary coil
Vs
Ns
or

Vp N p
• If Vs>Vp: step-up
transformer
• If Vs<Vp: stepdown transformer
Transformer example
• A transformer has 100
turns on the primary coil
and 300 turns on the
secondary coil. If 20V AC
is applied to the primary
coil, what will be the
voltage on the secondary
coil?
• A device is connected to
the secondary coil which
draws a current of 2 A.
What is the current
flowing in the primary
coil?
Vs N s

Vp N p
300 Vs
 ,
100 20
so Vs  60 V
I pV p  I sVs
I p  20  2  60
Ip  6A
Power in a transformer
Power in  Power out
so I pV p  I sVs
• So if a transformer steps up the voltage,
the current is stepped down.
– You can’t get something for nothing!
• This assumes an ideal transformer, where
no energy is lost to heating
Electricity transmission
• When electricity is transmitted over
power lines, some power is lost due
to the resistance of the cables (as
heat)
• P = IV and V=IR so Power lost = I2R
– So the higher the current, the more
power we lose
• A step-up transformer is used to
convert electrical power to very high
voltage (low current) for transmission
over long distances to minimise this
power loss
• It is converted back to a more useful
level at the other end by a step-down
transformer
Typical power transmission system
What’s it all for?
• Eg Maglev
train
(Shanghai)
• 432 km/h top
speed
• No moving
parts! (nearly)
Energy Generation
Electricity
Generation
• Can energy be generated?
– No! It can only be transferred from one form
to another
• What we mean is electricity generation
Industrial Electricity Generation
• Most of the world’s electricity is produced
in power stations:
1
2
3
4
Industrial Electricity Generation
• What fuels can be used?
• What are the relative advantages and
disadvantages of using those fuels in power
stations?
• What other ways are there of generating
electricity?
How else can we
turn turbines?
• Are they better or worse than power stations?
• How would you provide the electricity the
world wants?
Energy transfers
• eg wind power:
Kinetic
energy of air
Wind
Rotational
turbine kinetic energy
generator
Electrical
energy
• Draw energy transfer diagrams for:
•
Wave power, Fossil fuels, Tidal power, Hydroelectric
power, Geothermal power, Nuclear power,
Photovoltaic cells, Solar heating
Renewable or non-renewable?
• A non-renewable energy resource is one
which cannot be replaced once it has been
used.
• A renewable energy resource is one which
will not run out
• Categorise the following energy sources:
– Wind power, wave power, coal, tidal power,
oil, hydroelectric power, gas, geothermal
power, nuclear power, photovoltaic cells,
biomass (wood), solar heating
Industrial Electricity Generation
• What are the
advantages and
disadvantages of:
• Think about:
•
•
•
•
•
•
•
•
Cost?
Reliability?
Continuity of supply?
Easy to start and stop?
Renewable/non-renewable?
Access to fuel?
Waste products and
environmental impact?
Location?
•
•
•
•
•
•
•
•
•
Wind power
Wave power
Fossil fuels
Tidal power
Hydroelectric power
Geothermal power
Nuclear power
Photovoltaic cells
Solar heating
Energy
resource
Advantages
Disadvantages
6.2 understand that magnets repel and attract other magnets and attract magnetic substances
6.3 describe the properties of magnetically hard and soft materials
6.4 understand the term ‘magnetic field line’
6.5 understand that magnetism is induced in some materials when they are placed in a
magnetic field
6.6 describe experiments to investigate the magnetic field pattern for a permanent bar magnet and that
between two bar magnets
6.7 describe how to use two permanent magnets to produce a uniform magnetic field pattern.
6.8 understand that an electric current in a conductor produces a magnetic field round it
6.9 describe the construction of electromagnets
6.10 sketch and recognise magnetic field patterns for a straight wire, a flat circular coil and a
solenoid when each is carrying a current
6.11 understand that there is a force on a charged particle when it moves in a magnetic field as
long as its motion is not parallel to the field
6.12 understand that a force is exerted on a current-carrying wire in a magnetic field, and how this
effect is applied in simple d.c. electric motors and loudspeakers
6.13 use the left hand rule to predict the direction of the resulting force when a wire carries a current
perpendicular to a magnetic field
6.14 describe how the force on a current-carrying conductor in a magnetic field increases with the
strength of the field and with the current.
6.15 understand that a voltage is induced in a conductor or a coil when it moves through a magnetic
field or when a magnetic field changes through it and describe the factors which affect the size of the
induced voltage
6.16 describe the generation of electricity by the rotation of a magnet within a coil of wire and of a coil
of wire within a magnetic field and describe the factors which affect the size of the induced voltage
6.17 describe the structure of a transformer, and understand that a transformer changes the
size of an alternating voltage by having different numbers of turns on the input and output
sides
6.18 explain the use of step-up and step-down transformers in the large scale generation and
transmission of electrical energy
6.19 know and use the relationship between input (primary) and output
(secondary) voltages and the turns ratio for a transformer:
6.20 know and use the relationship: input power = output power
VP IP = VS IS for 100% efficiency
4.2 describe energy transfers involving the following forms of energy: thermal(heat), light, electrical,
sound, kinetic, chemical, nuclear and potential(elastic and gravitational)
4.16 describe the energy transfers involved in generating electricity using:
Wind, , water, geothermal resources, solar heating systems, solar cells, fossil fuels, nuclear power
4.17 describe the advantages and disadvantages of methods of large scale electricity
production from various renewable and nonrenewable resources.